The present invention is in the field of semiconductor manufacturing and more specifically relates to improvements in apparatus for handling semiconductor wafers as they are carried through a planarizing machine.
As supplied to the planarizing machine, the surface of a wafer may exhibit departures from flatness. Typically, 25 wafers are loaded into a cassette which is hand carried to a complex machine called a planarizer. The purpose of the planarizer is to render the front surface of the wafer flat to within a fraction of a micron. After each wafer has been rendered flat, the planarizer inserts it into a cassette. When the cassette is full, it may be removed from the planarizer and transported to any of a number of other processing machines.
The present invention cannot be fully appreciated without an understanding of the operation of the planarizer. Once the cassette containing typically 25 pre-processed wafers has been inserted into the planarizer, the operation of the planarizer in processing the wafers is completely autonomous. The operations are carried out under control of a computer that controls the application of power to various actuators in response to sensed inputs in a time-coordinated manner. In order for its lengthy program of operations to be carried out without mishap, it is important that means be provided to interrupt the accumulation of positional errors. It can also be appreciated that the wafers must be handled carefully to avoid scratching the processed surfaces which are so fragile that they are never again touched by human hands.
The present invention can best be understood by following a typical wafer as it is moved through the planarizer. The wafer is extracted from the cassette by a portion of the planarizer called a robot. The robot carries the wafer to a fixed location known as the load station. After the robot has deposited the wafer at the load station and has moved out of the way, a different part (called a spindle carrier) moves into position above the wafer and acquires it. The spindle carrier transports the wafer to a rotating polishing platen and rotates the wafer while holding it in contact with the rotating polishing platen until the wafer has been rendered flat. Thereafter, the spindle lifts the wafer from the polishing platen and transports it back to the load station where the wafer is rinsed. After the spindle carrier has deposited the wafer at the load station and has moved out of the way, the robot picks up the polished wafer and carries it to a cassette and deposits the wafer in the cassette. Throughout these operations, the wafer is maintained in a horizontal position with the side that is to be polished facing downward. That downward facing side of the wafer is also referred to as the front side of the wafer. The path of an individual wafer through the planarizer will be depicted in greater detail below.
Upon reflection it will be seen that the main purpose of the load station is to accurately center the wafer into the carrier notwithstanding the errors in their relative positions as they enter the load station.
In addition to centering the wafer with respect to the carrier, the load station performs a number of useful ancillary functions. The load station includes provision for sensing the presence of the wafer at the load station, for rinsing the wafer, for lifting the wafer into the carrier, and for supporting the processed wafer on cushions of water to avoid direct contact. Thus, the load station serves several other functions in addition to centering the wafer with respect to the carrier.
A wafer that has been deposited at the load station reposes in a horizontal attitude on cushions of purified water that are produced by three upwardly-directed nozzles affixed to a load ring, which is free to slide laterally on a horizontal base. Initially, the wafer is not centered within the load ring and the load ring is not centered with respect to the vertical axis of the spindle carrier. One objective of the present invention is to eliminate these centering errors.
In accordance with the present invention this is accomplished through the provision of a load ring assembly that is driven by the spindle carrier as it descends from an initial position above the load ring assembly. The load ring assembly includes, in addition to the load ring, at least three centering tabs spaced around the periphery of the load ring and hinged to the load ring by hinges having axes that are horizontal and tangential to the load ring. These centering tabs are yieldingly biased to an open position in which certain inwardly-facing surfaces of the centering tabs diverge upwardly. As the spindle carrier descends, it pushes downwardly on an upper surface of the load ring, overcoming the biasing force to cause the centering tabs to pivot in such a way that the aforementioned inwardly-facing surfaces converge to a vertical attitude. As the inwardly-facing surfaces converge, they contact the periphery of the spindle carrier, centering the load ring with respect to it, and they also contact the edge of the hovering wafer, centering the wafer with respect to the load ring. Because the wafer is centered with respect to the load ring and the load ring is centered with respect to the spindle carrier, it follows that the wafer is centered with respect to the spindle carrier.
In accordance with the present invention, the upwardly-directed nozzles that form the liquid cushions on which the wafer reposes are affixed to the load ring, so as not to disturb the cushions as the load ring is pushed downward by the descending spindle carrier.
The presence of the wafer on the liquid cushions produced by the upwardly-directed nozzles slightly impedes the flow of the purified water from the nozzles, causing an increase in the pressure in the conduits that feed the nozzles. In accordance with the present invention, this pressure increase is sensed by a pressure sensor and serves to indicate the presence of a wafer. When the wafer is absent, a lower pressure prevails, and the sensed lower pressure serves to indicate the absence of a wafer from the load station.
After the above-described centering procedure has been completed, the wafer must be acquired by the spindle carrier and rinsed. In accordance with the present invention, this is accomplished by the provision of a unique upwardly-directed nozzle located below the wafer and centered with respect to it. A spool-like portion of this nozzle discharges purified water against the underside of the wafer, rinsing it from the center outwardly, and lifting the wafer on a cushion of purified water. The spool-like portion then rises, carrying the wafer upward on the liquid cushion until the wafer comes into contact with a vacuum pad located on the underside of the spindle carrier, where the wafer is acquired by the suction of the vacuum pad, which holds the wafer to the spindle carrier.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.